Therapeutic approaches for sickle cell anemia include pharmacologic induction of fetal hemoglobin. Toward this end, we examine the influence of oxygen tension (pO2) on erythropoiesis. We utilize in vitro cultures of adult human hematopoietic progenitor cells cultured at varying oxygen tensions. Small decreases were observed in cell proliferation for 20-5% O2 with a marked 2-fold reduction at 2%, due in part to cytotoxicity. Minimal changes were detected in hemoglobin-containing cells at the end of the culture period. Reduced pO2 increased fetal hemoglobin and ganna-globin expression with maximal levels around 5% O2. Hemoglobinization accumulation and glycophorin A expression suggested a premature increase in erythroid differentiation. Cultures at reduced pO2 were characterized by early induction of gamma-globin and delayed and reduced peak levels of beta-globin expression and the transcription factors, EKLF, GATA-1 and SCL/Tal1. Changes in SCL/Tal1 correlated with overall globin expression, but did not appear to be preferential for gamma- or beta-globin. At reduced pO2 induction of erythropoietin receptor is also delayed and reduced, suggesting that the early increase of gamma-globin is a consequence of reduced erythropoietin signaling. These changes provide insight on the potential mode of action of chemical inducers of fetal hemoglobin.